Atmospheric CO₂ concentrations and δ¹³C values across the Antarctic Circumpolar Current between New Zealand and Antarctica

Abstract

Measurements of atmospheric CO₂ concentrations were repeatedly carried out on the vessel ‘Italica’ of the Italian National Research Program in Antarctica, during cruises from Italy to Antarctica. Discrete air samples were also collected in 4-L Pyrex flasks during these cruises in order to carry out δ¹³C analyses on atmospheric CO₂. The results acquired between New Zealand and Antarctica are reported here. The mean growth rate of the CO₂ concentration from 1996 to 2003 in this area of the Southern Oceans is of about 1.8 ppmv yr^-1, in good agreement with NOAA/CMDL measurements. The rates of increase from cruise to cruise are rather variable. From 1996.1997 to 1998.1999 cruise the yearly growth rate is 2.75 ppmv yr^-1, close to the large growth rates measured in several areas and mainly related to the most severe El Ni.no event of the last years. The other yearly growth rates are of about 1.3 and 2 ppmv for the periods 1998.1999 to 2001.2002 and 2001.2002 to 2003.2004, respectively. The large difference between these two values is probably related to the uncertainty on the only two 2001.2002 discrete measurements of CO₂ concentration in this area. The measured δ¹³C values show two completely different distributions and a large interannual variability. The 1998.1999, 2002.2003, and 2003.2004 results obtained between about 55.S and 65.S across the Antarctic Polar Front show a marked negativization of up to more than 2° when compared to the background values. The results are related to local source regions of CO₂, as frequently found in the Southern Ocean by several authors; the negative δ¹³C values are tentatively related to the possible contribution of different causes. Among them, the southward negative gradient of δ¹³C of the dissolved inorganic carbon, the contribution from upwelling deep waters and from subsurface processes between the Northern SubAntarctic Front and the Polar Front, and, partly, the contribution of CO₂ of biogenic origin, e.g. from heterotrophic activity. The 2001.2002 results are very homogeneous and almost constant, close to .8.5% showing only minor deviations from the oceanic background values. This behaviour may be related to differences in the frontal structure along the 2001.2002 track.